Bone marrow hematopoietic stem cells continually seed the blood lineages throughout adult life. E47 is a major transcription factor whose activity is critical for the progression of uncommitted hematopoietic progenitors to the lymphoid lineages. Perturbations in E47 expression are also associated with blood cancers. We have found that mice lacking E47 have a profound depletion of bone marrow multipotent progenitors as well downstream lineage restricted populations. Primitive precursors from E47 deficient mice exhibit heightened proliferative activity but decreased expression of key markers associated with differentiation. Reciprocal gain of function and loss of function studies identify two key genes as candidate E47 targets: the cell cycle regulator p21 and the transcriptional regulator Ikaros. Here, we propose to establish the mechanistic relationship between E47 and these two factors in primary bone marrow precursor subsets. In Aim 1, we focus on the proliferative defect and determine if E47 is a critical regulator of the balance between stem cell proliferation and self-renewal. In Aim 2, we examine whether E47 and p21 collaborate in the same genetic pathway to regulate stem cell proliferation. In Aim 3, we determine if Ikaros is the essential E47 target gene that regulates the differentiation of primary multipotent progenitors to the lymphoid lineages. Together, these aims address the molecular mechanisms that regulate the functional integrity of stem cells during lymphoid lineage progression. This work will provide new information about the molecular events that control stem cell integrity, and will provide key insight into the mechanisms by which dysregulation of E47 activity contributes to cancers of lymphoid origin. Public Health Relevance: The immune system is capable of responding to a vast array of potential pathogens. This is accomplished in part through the production of B and T lymphocytes. Our studies are aimed at elucidating the factors that regulate the functional integrity of hematopoietic stem cells during progression to the lymphocyte lineages.